1. Field of the Invention
The present invention relates to a radio frequency identification (RFID) tag. More particularly, the present invention relates to a batter-assisted tag in accordance with an EPCglobal class 1 generation 2 RFID standard.
2. Description of Related Art
As radio frequency identification (RFID) technique gradually becomes mature, and a price thereof decreases, application of the RFID technique is widespread. For example, a RFID tag is applied to a smart card (a chip card or an integrated circuit (IC) card) for providing convenience and security.
Generally, the RFID tags are mainly grouped into passive, semi-passive and active RFID tags according to different power supplies. The power supply of the passive RFID tag mainly comes from a received RF signal, and after a power of the RF signal converted, an operating power required by the passive RFID tag is provided. According to an EPCglobal standard, the passive RFID tag can be defined as a class-1 or a class-2 RFID tag, the class-1 RFID tag is belonged to an identify tag, and the class-2 RFID tag is belonged to a higher-functionality tag.
The semi-passive RFID tag has a battery for supplying the required operating power. However, the same as the passive RFID tag, the semi-passive RFID tag is belonged to a passive communication and uses a received RF signal as a carrier of a modulated backscatter signal thereof. The semi-passive RFID tag is also referred to as a battery-assisted passive tag, which requires a battery to provide the operating power. However, when the battery is out of power, the semi-passive RFID tag cannot be normally operated.
Since the passive RFID tag does not require the battery, a usage lifespan thereof is longer than that of the active or semi-passive RFID tag. However, since the operating power thereof comes from the RF signal sent by a reader, an effective read distance thereof is relatively short. Meanwhile, due to an influence of a multipath fading effect, a power of the signal received by the passive RFID tag is liable to be less than the required operating power.
Moreover, when the passive RFID tag in a wireless ID system receives a reader command, a demodulator of the RFID tag down-converts the RF signal transmitted by the reader to generate a base band signal for decoding. When the RFID tag decodes the reader command, the signal thereof has to satisfy a demand of a minimum modulation depth that can be correctly interpreted by the RFID tag, so that the RFID tag can correctly decode the reader command. However, in a multi-carrier environment, a minimum amplitude difference required for receiving the reader command limits a performance of the tag read distance.